Electronic control system



Nov. 21, 1944. D. P. FAULK 2,363,305

ELECTRONIC CONTROL SYSTEM Filed 06.7"u 30, 1942 WITNESSES:

INVVENTOR 41 J DonaMPlZu/K, Z- B I AT.T EY

Patented Nov. 21, 1944 ELECTRONIC CONTROL SYSTEM Donald P. Faulk,Pittsburgh, Pa., assignor to Westinghouse Electric & ManufacturingCompany, East Pittsburgh, Pa., a corporation Pennsylvania ApplicationOctober 30, 1942, Serial No. 463,989

'7 Claims.

This invention relates to an electronic control system and hasparticular relation to a system in which power is supplied from analternating potential source to a load through a pair of inverselyconnected electric discharge valves.

In many systems in which the supply of current to a load is controlledby inversely connected electric discharge valves, it is highly desirableto employ a single source of control potential for both valves. In aresistance welding system, for example. the fiow of welding current issupplied through a pair of inversely connected ignitrons. A firingcircuit is provided for each ignitron and includes an electric dischargevalve, preferably a thyration, connected between the anode and igniterof the corresponding ignitron, so that the firing valves themselves areinversely connected in circuit with the source. The firing valves arethen controlled to render the ignitrons conductive alternately insuccessive half periods of the source. A control system is arranged toprovide a pulsating control potential of suitable wave form forrendering the firing valves conductive at a selected instant in eachhalf period for a predetermined interval of time. It is obviouslydesirable to employ a single control potential for both firing valvesrather than provide a second control system. However, if a single sourceof control potential is to be connected inthe control circuit of bothfiring valves, the grids of the valves must be connected together andthe oath odes must be connected together. To avoid a short circuitacross the firing valves, a large impedance must be inserted in theconnection be tween the cathodes. As a result, an alternatingpotentialap-pears across this impedance which interferes with theimpressing of a control potential of suitable wave form between thegrids and cathodes of firing valves. I

Different arrangements have been employed in the past in an attempt tocounteract or nullify the alternating potential appearing across theimpedance which is connected betweenthe cathodes of the firing valves.One way of accomplishing this result is to employ an inversely connectedtransformer to impress another alternating potential in the controlcircuit which is 180 out of phase with the source potential. However. inactual practice where current is conducted throughthe valves during onlya portion of each half period, the alternating potential across theimpedance between the cathodes is not a sine wave and consequently analternating potential of sine wave form 180 out of phase with.

the source cannot effectively remove the undesired potential from theimpedance.

Another method of counteracting the alter hating potential appearingacross the impedance which is connected between the cathodes of thefiring valves employs a pair of gaseous discharge rectifier tubesconnected back-to-back across the impedance. The junction point betweenthe two rectifiers is then connected to a center tap on the impedance,the rectifiers being arranged to conduct current away from the centertap. The control potential of suitable wave form is then impressedbetween the grids of the firing valves and the center tap on theimpedance. In a halfperiod of one polarity, one of the rectifiersshortcircuits the one-half of the impedance in the control circuit ofthe firing valve which is in condition to becomeconductive. During theoppcsite half period, the other rectifier short-cir cuits the other halfof the impedance which is in the control circuit of the other firingvalve.

The prior art control arrangement just described operates satisfactorilyon low voltage circuits. However, when a high voltage is employed, onerectifier becomes conductive just as the other has ceased to conductcurrent and the back potential appearing across the nonconductingrectifier causes it to backfire. As the conducting rectifier hasextremely low resistance, a very large current passes through thebackfiring rectifier and causes considerable damage thereto and disruptsthe control of the firing valves.

It is, accordingly, an object of my invention to provide a new andimproved system for supplying power from an alternating potential sourceto a load through a pair of inversely connected electric dischargevalves in which a single control potential source is connected incircuit between the control electrode and cathode of both valves.

Another object of my invention is to provide a novel system forsupplying power from an alternating potential source to a load through apair of inversely connected electric discharge valves of the arc-liketype in which a single source of periodically pulsating controlpotential is connected in circuit between the control electrode andcathodeof both valves.

More specifically, it is an object of my invention to provide anew andimproved system for supplying power from an alternating potential sourceto a load through a pair of inversely connected electric dischargevalves controlled by a single source of control potential in which thealternating potential is eifectively removed from the control circuitwithout danger of breakdown because of high voltage.

In accordance with my invention, a first, second and third impedancemeans is connected in series in the order named between the cathodes ofthe firing valves. A first gaseous rectifier discharge tube is connectedto efiect short-circuiting of the first and second impedance means whenthe valve whose cathode is connected to the first impedance means has apositive anode potential. A second gaseous discharge rectifier tube isconnected to short-circuit the second and third impedance means in theopposite half period of the source. The second impedance means may beshort-circuited by either rectifier. The control potential is thenimpressed between the control electrode of the firing valves and acenter tap on the second impedance means.

When one of the firing valves has a positive anode potential, one of therectifiers becomes conductive to short-circuit the impedance in thecontrol circuit of that valve. The rectifier does not, of course, have azero arc drop potential. However, the rectifier arc. drop potential isso low that the voltage thereacross is very small, of'the order of 10volts regardless of the magnitude of current thereto. Consequently asubstantially constant residual potential of 10 volts appears across thetwo impedance means shorted by the rectifier but as the controlpotential is impressed between the :control electrode of the firingvalve and the center tap of the second impedance means, the residualpotential remaining in the control circuit is of the order of 6 volts,which is negligible for all practical purposes.

At the end of the first half-period the first rectifier ceases toconduct andthe other rectifier is rendered conductive to nullify thealternating potential in the control circuit of the second firing valve.It is to be noted that the second impedance means is connected betweenthe two rectifiers. Then, if the first rectifier backfires, the currenttherethrough is limited to a low value because of the second impedancemeans in series therewith. Consequently, there is very little danger ofpermanent disruption of the control system by a backfiring of one of therectifier tubes.

The novel features that I consider characteristic of my invention areset forth with particularity in the claims. The invention itself,however, both as to its organization and it method of operation,together with additional objects and advantages thereof, will best beunderstood from the following description of a specific embodiment whenread in connection with the accomof the source. The secondary H of thewelding transformer i3 is connected across a pair of welding electrodesl9 and 2| in engagement with the material 23 to be welded. The firingcircuit for one of the ignitrons I may be traced from the terminal I 5of the source 3, through the anode 25 and cathode 21 of an electricdischarge firing valve 29, the igniter 3| and cathode 33 of the ignitronand the primary H of the welding transformer l3 to the other terminal 501 the source. The firing circuit for the other ignitron 9 may be tracedfrom the terminal 5, through the primary II, the anode 35 and cathode 31of a second electric discharge firing valve 39, the igniter 4| andcathode 43 of the second ignitron 9 to the other terminal I5 of thesource. It is thus apparent that the firing valves 29 and 39 are alsoconnected inversely or in antiparallel in circuit with the source. Thefiring valves are of the arc-like type, preferably thyratrons, and theircontrol electrodes 44 and 45 are interconnected through correspondinggrid resistors 46 and 47 and a conductor 49. The cathodes 21 and 31 ofthe firing valves 29 and 39 are also interconnected through first,second and third resistors 5|, 52 and 53. A gaseous discharge rectifiertube 55 is connected across the first and second resistors 5| and 52 anda second gaseous discharge rectifier tube 51 is connected across thesecond and third resistors 52 and 53. The rectifier tube 55 is arrangedto conduct current away from the junction point of the first and secondresistors 5| and 52, and rectifier tube 51 is arranged to conductcurrent away from the junction point of the second and third resistor 52and 53.

A control system 59 is provided to supply a control potential forcontrolling the firing valves 21 and 31. The control system includes avoltage divider 6| energized from the source 3 through an auxiliarytransformer 63, a full wave rectifier 65 and filtering elements 51. Anintermediate tap 69 on the divider 6| is connected to a center tap H Ion the second resistor 52 interconnecting the cathodes of the firingvalves 29 and 39. The negative terminal 13 of thedivider 6| is connectedto the control electrodes 44 and 45 of the firing valves, The circuitfrom the negative terminal 13 of the divider El may be traced throughresistors 15, 11 and 19, a balancing potentiometer circuit 8|, conductor82, conductor 49 and resistor 46 to control electrode 44 or resistor 4|to control electrode'45. As will be explained hereinafter, theimpressing of a control potential between the control electrodes 44and'45 of the firing valves and the center tap H is the same asimpressing the control potential directly between the control electrodeand cathode of the firing valve whose anode is positive at the time.

If ignitrons and 9 are identical and firing valves 29 and 39 areidentical, the balancing potentiometer circuit 8| does not supply apotential in the circuit. Therefore, function of the balancingpotentiometer may be omitted from immediate consideration and will beexplained hereinafter.

A potential is impressed across the resistor I9 in the circuit betweenthe negative terminal 13 of the divider 6| and the control electrodes ofthe firin valves. This potential is derived from the source 3 through anauxiliary transformer 83 a phase shifting circuit 85 and a full waverectifier 81, so that the potential takes the form of a rectifiedalternating potential, inverted with respect to the controlelectrodesand displaced in phase relative to the source.

A direct-current potential is impressed across the other resistor 1! inthe circuit between the negative terminal 73 of the divider 6| and the'Portion of the divider 81 between the intermediate tap $9 and thenegative terminal 13. Because of the high negative direct-currentpotential supplied from the divider SI and the resistor 11, theresultant potential impressed between the control electrodes of thefiring valves 29 and 39 and the center tap of the second resistor 52remains below the critical control electrode-cathode potential necessaryto render the firing valves conductive.

Another electric discharge valve 93 of the arclike type, preferably athyratron hereinafter designated as the start valve, is connected acrossthe voltage divider GI in a circuit extending from the positive terminal95 through a contactor 91 of a push-button switch 99, the anode IN andcathode I93 of the start valve 93 and parallel circuits comprising theresistor 15 on one side and a rectifler I05, potentiometer I01 andcapacitor I99 on the other side to the negative terminal 13 of thedivide: GI. The control circuit of the start valve 93 may be traced fromits control electrode III, through a grid resistor H3, a resistor H5,and resistor 11 to the cathode I03. As previously set forth, adirect-current potential is impressed across the resistor 11 and thispotential is of such polarity as to tend to maintain the start valve 93nonconductive. Potential impulses are impressed across the otherresistor I I5 in the control circuit of the start valve through apeaking transformer H1 and a phase shifting circuit II9 energized fromthe source. The phase shifting circuit H9 is arranged to be adjustedaccording to the power factor of the load so that an impulse across theresistor II5 occurs at the beginning of each half-period of alternatingcurrent. The polarity and magnitude of the impulse across the resistorII5 at the beginning of a positive halfperiod of the source current issuch that the impulse overcomes the biasing potential across theresistor 11 and renders the start valve 93 conductive if the push-buttoncontactor 91 is closed.

When the start valve 93 becomes conductive, the upper end of theresistor 15 in series therewith is effectively connected to the positiveterminal 95 of the divider 6!. Consequently, the direct-currentpotential supplied from the divider to the circuit between the controlelectrodes of the firing valves and the center tap H of the secondresistor 52 becomes considerably less negative. As a result, the peaksof the inverse rectified potential in the circuit rise above thecritical control electrode-cathode potential necessary to render thefiring valves conductive. This inverse rectified alternating potentialis employed to determine the instant at which the firing valves 29 and39 and, therefore, the ignitron 1 and 9 become conductive in ahalf-period of the source. The particular instant is selected byadjustment of the phase shifting circuit 85 to shift the position of thepeaks and is effective to determine the amount of current supplied tothe weld in a halfperiod, which, in turn, determines the character ofthe weld.

A second electric discharge valve I2l of the arc-like type, preferably athyratron, which is hereinafter designated as a stop valve, is connectedbetween the positive terminal 95 of the divider 6| and anotherintermediate tap I23 on the divider. The control circuit of the stopvalve I2I may be traced from the control electrode I25 thereof through agrid resistor I21 and the capacitor I09 to the negative terminal 13 ofthe divider BI and from the intermediate tap I23 to the cathode I29 ofthe stop valve. The potenfirst firing valve 29.

tlal between the negative terminal 13 and intermediate tap I23 of thedivider normally biases the stop valve to maintain it nonconductive.However, when the start valve 93 is conductive the capacitor I09 inseries therewith is charged at a rate determined by the setting of thepotentiometer I01. After a predetermined interval of time, the potentialacross the capacitor I09 rises to a suflicient magnitude to counteractthe biasing potential in the control circuit of the stop valve I2I andrender the stop valve conductive. When the stop valve I2I becomesconductive, the intermediate tap I23 is effectively connected to thepositive terminal of the divider so that the upper end of resistor 15 isagain highly negative with respect to intermediate tap 59. A backcontactor I3I on the push-button switch 99 is arranged to complete adischarge circuit for the capacitor I09 when the push-button switch isreleased.

From the foregoing description it is apparent that the potential betweenthe control electrodes of the firing valves 29 and 39 and the center tapH of the second resistor 52 between the cathodes 21 and 31 originallycomprises an inverse rectified alternating potential superimposed on ahighly negative direct-current potential of such value that theignitrons are not rendered conductive. When the push-button contactor 91is closed, the direct-current potential becomes less negative while theinverse rectified alternating potential remains the same and theignitrons are rendered conductive alternately at-a selected instant insuccessive half-periods. After a preselected interval of time determinedby the setting of the potentiometer I 01, thedirect-current potentialagain becomes highly negative and the flow of current through theignitrons is halted.

A careful consideration of the control circuits .for the firing valvesreveals that the impressing is positive at the time.

The three resistors 5|, 52 and 53 interconnecting the cathodes 21 and31. of the firing valves 29 and 39 are also connected across the source3 through the igniters 3| and M and cathodes 33 and 43 of the ignitrons1 and 9. Consequently, an alternating potential in phase with the sourceappears across these resistors. When the potential across the resistors5|, 52 and 53 is of such polarity that the cathode connected end of thefirst resistor 5] is negative, the first rectifier 55 becomes conductiveto short-circuit the first and second resistors 5| and 52. It followsthat the control potential impressed between the control electrodes 44and 45 of the firing valves 29 and 39 and the center tap H of the secondresistor 52 is then effectively impressed directly between the controlelectrode 44 and cathode E1 of the Similarly, when the cathode connectedend of the third resistor 53 is negative, the second rectifier 51becomes conductive to short-circuit the second and third resistors 52and 53 and the control potential is effectively impressed between thecontrol electrode 45 and cathode 31 of the second firing valve 39.Interference with the shape of the wave form of the control potential bythe alternating potential across the three resistors is thus avoided. Asa result, the control potential may be adjusted to rise above thecritical value of the firing valves at any selected instant in ahalf-period and the ignitrons are rendered conductive at that selectedinstant.

Should the igniting characteristic of the two ignitrons be slightlydifierent, the balance potentiometer 8! may be adjusted to add analternating potential component to the control potential to raisealternate peaks of the inverse rectified alternating potential. As aresult, the two ignitrons may be conductive for the same percentage of ahalf-period.

In a system connected as illustrated and tested, the ignitrons and 9were Westinghouse WL-656 tubes, and the firing valves 29 and 39 wereWestinghouse Wit-632 thyratrons. Each of the three resistors 52 and 53had a resistance of 15,000 ohms and the gaseous discharge rectifiers 55and 51 were Westinghouse WL-866A-866 tubes.

Although I have shown and described a preferred embodiment of myinventionQI am fully aware that many modifications thereof are possible.My invention, therefore, is not to be restricted, except in so far as isnecessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. For use in supplying power from a source of alternating potential toa load, the combination comprising a pair of electric discharg valves,each having an anode and cathode connected between said source and loadand a control electrode, said valves being connected in anti-parallel, afirst, second and third impedance means connected in series in the ordernamed between the cathodes of said valves whereby an alternatingpotential in phase with said source appears across said three impedancemeans, means responsive to a potential across said three impedance meansof such polarity that the cathode connected end of said first impedancemeans is negative for short circuiting said first and second impedancemeans, means responsive to a potential across said three impedance meansof opposite polarity for short circuiting said second and thirdimpedance means, and means for impressing a control potential betweensaid control electrodes and an intermediate tap on said second impedancemeans.

2. For use in supplying power from a source of means for shortcircuiting a second section of said impedance means, said first andsecond sections having a common portion with each section including partof said impedance means between said common portion and the cathodewhich is negative when the corresponding section is short circuited, andmeans for impressing a control p0- tential between said controlelectrodes and an intermediate tap on said common portion,

3. For use in supplying power from a source of alternating potential toa load, the combination comprising a pair of electric discharge valves,each having an anode and cathode connected between said source and loadand a control electrode, said valvesbeing connected in anti-parallel, afirst,

second and third impedance means connected in series in the order namedbetween the cathodes of said valves, first rectifying means connectedacross said first and second impedance means in a manner to conductcurrent away from the junction of said second and third impedance means,second rectifying means connected across said second and third impedancemeans in a manner to conduct current away from the junction of saidfirst and second impedance means, and means for impressing a controlpotential between said control electrodes and an intermediate tap onsaid second impedance means.

4. For use in supplying power from a source of alternating potential toa load, the combination comprising a pair of electric discharge valves,each having an anode and cathode connected between said source and loadand a control electrode, said valves being connected in anti-parallel, afirst, second and third impedance means connected in series in the ordernamed between the cathodes of said valves, first rectifying means of thegaseous discharge type connected across said first and second impedancemeans in a manner to conduct current away from the junction of saidsecond and third impedance means, second rectifying means of the gaseousdischarge type connected across said second and third impedance means ina manner to conduct current away from the junction of said first andsecond impedance means, and means for impressing a control potentialbetween said control electrodes and an intermediate tap on said secondimpedance means.

5. For use in supplying power from a source of alternating potential toa load, the combination comprising a pair of electric discharge valvesof 1 the arc-like type, each having an anode and cathode connectedbetween said source and load and a control electrode, said valves beingconnected in anti-parallel, a first, second and third impedance meansconnected in series in the order named between the cathodes of saidvalves, first rectifying means connected across said first and secondimpedance means in a manner to conduct current away from the junction ofsaid second and third impedance means, second rectifying means connectedacross said second and third impedance means in a, manner to conductcurrent away from the junction of said first and second impedance means,and means for impressing a periodically pulsating control potentialhaving a selected phase relation to said source between said controlelectrodes and an intermediate tap on said second impedance means.

6. For use in supplying power from a source of alternating potential toa load, the combination comprising a pair of electric discharge valvesof the arc-like type, each having an anode and cathode connected betweensaid source and load and a control electrode, said valves beingconnected in anti-parallel, impedance means connected between thecathodes of said valves whereby an alternating potential in phase withsaid source appears across said impedance means, means responsive to apotential of one polarity across said impedance means for shortcircuiting a first section of said impedance means, means responsive toa potential of opposite polarity across said impedance means for shortcircuiting a second section of said impedance means, said first andsecond sections having a common portion with each section including partof said impedance means between said common portion and the cathodewhich is negative when the corresponding section is short circuited, andmeans for impressing a periodically pulsating control potential having aselected phase relation to said source between said control electrodesand an intermediate tap on said common portion.

"7. For use in supplying power from a source of alternating potential toa load, the combination comprising a pair of electric discharge valvesof the arc-like type, each having an anode and cathode connected betweensaid source and load and a control electrode, said valves beingconnected in anti-parallel, a first, second and third resistance meansconnected in series in the order named between the cathodes of saidvalves, first rectifying means of the gaseous discharge type DONALD P.FAULK.

